Why do water bottles cave in after the expiration date?

Water bottles may cave in after the expiration date due to a combination of factors, including changes in temperature and atmospheric pressure. As the water inside cools or as the bottle is exposed to lower pressure, the air inside can contract, creating a vacuum that causes the sides of the bottle to collapse. Additionally, the plastic material can become less rigid over time, exacerbating this effect. However, it's important to note that the water itself does not expire; the date is more about quality and safety regarding the bottle's material.